This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-11-106500 filed on Apr. 14, 1999 and No. Hei-2000-39642 filed on Feb. 17, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a driver circuit for driving a display panel that includes plural capacitive elements such as electroluminescent elements, optical characteristics of which change according to voltages imposed thereon.
2. Description of Related Art
An electroluminescent display panel (referred to as an EL display panel) that includes a number of capacitive electroluminescent elements (referred to as EL elements) is driven by selectively charging and discharging the EL elements. A relatively large current flows in a scanning electrode driving circuit that supplies scanning voltages to scanning electrodes in such an EL panel, and a relatively large amount of power is consumed mainly as a switching loss in the scanning electrode driving circuit. Therefore, there has been a possibility that the scanning electrode driving circuit malfunctions due to heat generated by power loss.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved driver circuit which consumes less power and operates always stably.
A display panel having plural capacitive display elements is driven by supplying a driving voltage through a driver circuit. The driving circuit includes an integrated circuit having plural output circuits, each connected to each scanning electrode corresponding to the capacitive display elements aligned along the scanning electrode, and voltage dividing loads. The voltage dividing loads such as power transistors or resistors are connected between the power source and the integrated circuit. Each output circuit in the integrated circuit is composed of a first switching element for charging the capacitive display element and a second switching element for discharging the same, both constituting a push-pull circuit. LDMOS transistors or SCRs are used as the switching elements.
The voltage between the power source and the display panel is divided between the voltage dividing loads and the integrated circuit, and accordingly power consumption is also allocated to the voltage dividing load and the integrated circuit. Therefore, temperature rise in the integrated circuit is suppressed, and malfunction of the integrated circuit due to the temperature rise therein is avoided. When resistors are used as the voltage dividing loads, the power consumption in the circuit is allocated to the resistors and the integrated circuit according to respective impedance thereof.
When the power transistors such as MOSFETs are used as the voltage dividing loads, they are closed or opened after the switching elements in the integrated circuit are closed. Therefore, switching loss occurs only in the power transistors, avoiding switching loss in the integrated circuit. In this manner, the temperature rise in the integrated circuit is further suppressed, and the integrated circuit stably operates without being affected by the temperature rise.
Preferably, a step-recycle circuit including a condenser is connected between the power source and the power transistors. The level of the driving voltage supplied to the capacitive display elements is changed stepwise by charging and discharging the condenser. The power for driving the capacitive display elements are saved in this manner. At the same time, the condenser is used as a power recycling element. A portion of the energy charged in the capacitive display element is returned to the condenser in the step-recycle circuit and is used again to charge the element in the next cycle. The driving energy is further saved by recycling the energy in the capacitive display element.
Preferably, the display panel is driven with positive and negative voltages alternatively supplied thereto. In this case, a first and a second condenser are disposed in the step-recycle circuit. The first condenser functions to generate the stepwise changing driving voltage and to recycle the energy in the capacitive display element when the panel is driven with the positive voltage. The second condenser performs the same functions when the panel is driven with the negative voltage.
According to the present invention, the amount of power consumed in the integrated circuit is reduced, and thereby malfunctions of the integrated circuit due to temperature rise are avoided.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.